Epidemological and experimental studies have implicated high dietary fat in the increased incidence of mammary cancer. The fact that changes in dietary intake of lipids is reflected by comparable changes in the lipid matrix of the mammary gland suggests that fatty acids may have a direct role in mammary carcinogenesis. Experimental systems have been established for assessing fatty acid effects of the growth of cells from normal tissue and carcinogen induced mammary tumors of rats. The studies show that all unsaturated fatty acids present in the gland in appreciable amounts promote cell growth. Each fatty acid has its own characteristic concentration optima, which with normal mammary cells, correlates with the abundance of that fatty acid in the gland. Both normal and tumor cells are inhibited by saturated fatty acids. Maximum growth rate of tumor cells is obtained with oleic acid while normal cells grow best in the presence of linoleic or linolenic acid.